Is it possible to have an orbital satellite replenish the propellant for its ion engine by scooping it from the atmosphere?What are the key considerations for this?What orbital range are you restricted to?Are there any proposed experiments on the horizon, to prove such an idea?

Could this technique be useful around any other bodies, besides the Earth? If so, what missions could it benefit?

I've read about ion-propelled satellites having been put up that fly so low that they need to have some aerodynamic features to reduce atmospheric resistance, but they didn't scoop anything up. I've also read about ideas for hypothetical nuclear-powered scramjets driven by MHD that would suck in thin ionized air and expel it out the back propulsively, but they didn't use feeble ion drives.

There wouldn't be much air to "scoop" in LEO, to get a good amount you'd probably want to be in the lower atmo.

Xenon is pretty rare in the atmo, so since there's very little air in LEO, there's almost no Xenon up there.

Does the gas have to be Xenon?

As I understand it, most if not all ion drive craft can run on a wide variety of gases. You just have to select the one best suited to your needs, and is the least reactive with your drive system. (It would suck to corrode your ion drive using the wrong gas).

Here's one major problem. If you dip your perigee into the atmosphere, the drag is going to decrease your apogee. And your next perigee is going to be back in the atmosphere again.

The only way to raise your perigee out of the atmosphere is with a burn at apogee. But if you're using an ion drive, whch uses lowthrust over an extended time, you're not going to be able to manage a significant perigee boost before you're heading back down again. Your orbit is going to quickly degrade, until you end up re-entering and burning up.

I'm sure it's more subtle than that. If you had a VASIMR type drive that went from high efficiency near perigee, boosting time at apogee, to high thrust at apogee, booting height of perigee an equilibrium could be found.

It could also be possible to scoop mass from the atmosphere in the process.

Anything with protons can be ionized(i.e. everything made out of atoms) and then controlled with magnetic fields. Different gases work better as fuel than others because of density and difficulty in stripping electrons.

We will see how the recently launched SLATS experimental satellite goes. That would be a significant step in showing you can maintain orbit in lower parts of the atmosphere for enough time to start mining the upper atmosphere. Next step would be to mount hardware on the front of a similar satellite that would liquefy and divert gas to different storage tanks. Perhaps the heat exchanger technology being developed for SABRE would be useful for that. The exhaust velocity of even low isp ion engines is far above earth orbital velocity, so you should be able to gain more gas than is required to be used to maintain velocity. Even for Jupiter with an orbital speed of ~42 km/s, that would correspond to a ion engine isp of ~4000 to maintain velocity. Anything above that and you should be able to gain gas faster than you consume it.

Here's one major problem. If you dip your perigee into the atmosphere, the drag is going to decrease your apogee. And your next perigee is going to be back in the atmosphere again.

The only way to raise your perigee out of the atmosphere is with a burn at apogee. But if you're using an ion drive, whch uses lowthrust over an extended time, you're not going to be able to manage a significant perigee boost before you're heading back down again. Your orbit is going to quickly degrade, until you end up re-entering and burning up.

So, basically, the answer is "No."

Right, the aerobrake delta-v loss must be countered with a delta-v gain before next planetary approach, to raise periapsis. Since you're running an ion engine, continuous thrust would be applied. You might try scripting that sort of mission plan in GMAT, to see whether the required ISP is feasible.

One other possibility: If you controlled the aerobrake very precisely, you could conceivably plan your trajectory to perform a lunar close-approach, raising periapsis via gravity-assist. Earth's Moon would give a deflection and gravity-assist for up to ~1.7 km/s delta-v. That also would be something to see in GMAT.

So it sounds like a limiting case can be calculated based on planetary mass / escape velocity, solar flux, etc.

The lighter the planet (or moon) that you're orbiting, the heavier its ions are going to have to be. The closer you are to the Sun, the better the solar flux you'll get per unit spacecraft mass - unless somehow the solar array is separate and physically detached from the ion-driven spacecraft, sending power by microwave beam, for example.

Venus is slighter lighter and has much better solar flux - if this approach could work for Earth, then it would likely work for Venus. But Venus doesn't have the magnetic field and associated ionosphere - so does that affect the composition and availability of ionic species?

Atomic oxygen tends to destroy things. Ion drives move charged ions in a controlled way. When a pair of single ionized atoms become one double ionized atom and one neutral atom the neutral atom can ignore both your magnetic fields and electric fields. The ion drive has to be firing ions much faster than orbital velocity or you lose momentum scooping. Hot (high velocity) atomic oxygen should etch away the wall fairly quickly.

A grid type ion engine needs a relatively non reactive propellant that is easily ionized which is why xenon is the propellant of choice.

Now a VASIMR engine might be able to be made to work with a crude mix of gases that contain undesirable stuff like oxygen,water, and CO2 since there are no grids and the plasma is magnetically contained.The same goes for an arcjet rocket but in both cases engine would have to be designed to deal with this from the start.Though the concept had been thought of before with PROFAC.http://www.bisbos.com/space_n_profac.htmlThe oxygen was kept for use for refueling conventional rockets and the nitrogen was used in the electric engines to keep it in orbit.

We've looked at these air-breathing electric propulsion systems extensively. The ASPW (advanced space propulsion workshop) has featured several of these concepts, ours was a number of years ago.

My view was this was tough, but maybe not impossible. As others have noted, the orbital mechanics is pretty easy as long as you 'burn' as you collect, so that you are at least counteracting your drag. Hall Effect, FRC, and Helicon thrusters have all shown to work on some combination of N2 and O2 (and maybe stored Xenon for the cathode). The basic idea as we understand it is you need an Isp that is atleast the orbital velocity divided by all of your propulsive/collection efficiencies to make up for drag and more if you want to store. At 100% efficiency and assuming your vehicle was a flow-through 'scoop' and stored nothing, you'd only need 800-ish seconds.

But, with a 50% collection and pressurization efficiency, a 75% thruster mass utilization efficiency, and storing 25% of the gas each orbit you need an ISP of 2700 seconds. This assumes the entire front end of this thing is all scoop.

I think the biggest concern is heat rejection and power. If your spacecraft is much bigger than the scoop/inlet (ie solar panels) the amount of thrust needed to counteract all of the drag gets a lot harder. And while the thruster puts most of its energy into the propellant, either heat or kinetic, the scoop doesn't have a natural way to loose energy.

...you 'burn' as you collect, so that you are at least counteracting your drag. Hall Effect, FRC, and Helicon thrusters have all shown to work on some combination of N2 and O2... you need an Isp that is at least the orbital velocity divided by all of your propulsive/collection efficiencies to make up for drag and more if you want to store...

Hmm. Typically those thruster designs give a few Newtons of thrust, at most. I don't understand how they could be used to counteract drag as you collect, unless the collection altitude were extremely high, and collection mass extremely small; setting an impractically low limit on collection. Can you clarify with some more quantitatives?

We've looked at these air-breathing electric propulsion systems extensively. The ASPW (advanced space propulsion workshop) has featured several of these concepts, ours was a number of years ago.

My view was this was tough, but maybe not impossible. As others have noted, the orbital mechanics is pretty easy as long as you 'burn' as you collect, so that you are at least counteracting your drag. Hall Effect, FRC, and Helicon thrusters have all shown to work on some combination of N2 and O2 (and maybe stored Xenon for the cathode). The basic idea as we understand it is you need an Isp that is atleast the orbital velocity divided by all of your propulsive/collection efficiencies to make up for drag and more if you want to store. At 100% efficiency and assuming your vehicle was a flow-through 'scoop' and stored nothing, you'd only need 800-ish seconds.

But, with a 50% collection and pressurization efficiency, a 75% thruster mass utilization efficiency, and storing 25% of the gas each orbit you need an ISP of 2700 seconds. This assumes the entire front end of this thing is all scoop.

I think the biggest concern is heat rejection and power. If your spacecraft is much bigger than the scoop/inlet (ie solar panels) the amount of thrust needed to counteract all of the drag gets a lot harder. And while the thruster puts most of its energy into the propellant, either heat or kinetic, the scoop doesn't have a natural way to loose energy.

So clearly you can do it, but there are still a number of challenges including building an ion thruster with the necessary performance, being able to collect the gas at such low pressure and refining the gas into something usable.

helicon/VASIMR is the most prominent electric (in the conventional sense) drive, though the original PROFAC scooper concept used a nuclear reactor driven resistojet thruster. Choice of orbit matters in other interesting ways, such as a solar powered scooper in a terminator riding SSO orbit would be continuously lit by the sun (providing power all the time, but also heat all the time). An interesting alternative is beaming laser power from the earth from 4 or so laser stations.

5 March 2018In a world-first, an ESA-led team has built and fired an electric thruster to ingest scarce air molecules from the top of the atmosphere for propellant, opening the way to satellites flying in very low orbits for years on end.

ESA’s GOCE gravity-mapper flew as low as 250 km for more than five years thanks to an electric thruster that continuously compensated for air drag. However, its working life was limited by the 40 kg of xenon it carried as propellant – once that was exhausted, the mission was over.

Replacing onboard propellant with atmospheric molecules would create a new class of satellites able to operate in very low orbits for long periods.